The production of metal parts utilizing powdered materials comprises a variety of techniques. One such technique, injection molding, offers a number of advantages over what may be termed "conventional" powder metallurgy methods. These advantages include comparatively low capital investment and operating costs, coupled with the ability to produce intricate metal shapes approaching the properties and densities of wrought metal, to "near-net shape", i.e., often obviating the need to perform secondary machining operations. The process is basically an offshoot of conventional plastic forming techniques and comprises:
(i) intimately mixing fine metal powders with an organic binder utilizing about 75-95 weight percent (or about 30-75 volume percent) metal powder--the balance, organic binder materials;
(ii) the so-mixed plasticized batch is then injection molded in conventional plastic molding equipment at relatively low pressures and temperatures into a mold generally designed to be about 10-20% larger in each dimension than the final desired part;
(iii) the organic binder is thereafter removed by solvent extraction or thermal degradation so as to not destroy the integrity of the shape; and
(iv) the resulting metal skeleton is thereafter sintered to final density, desirably approaching theoretical density.
In an attempt to provide a sintered part with superior mechanical properties and densities, the art has resorted to the use of sintering temperatures at the high end of the normal range, or longer hold times, or a combination of both. Since sintering is a diffusion-controlled phenomena, there is obviously a time/temperature relationship in achieving essentially equivalent sintered properties. Longer hold times are less desirable, because of increased cost resulting from extra energy usage and decreased productivity. While hold time can be decreased by resorting to higher temperatures, this expedient also offers a number of disadvantages in that it, too, is more costly--due to both higher energy costs and the need for more expensive furnaces.